Filling new field with Default Value

I will be using my current database for a new company. But want to retain all the current records identifying them with my old company. I have created a new field, added it to the contact record, given it a pull-down menu of only the old and new company names. Now I want to have my 900 records automatically populated with the old company name, and then I will add new contacts with the option of using old company or new company.

How do I populate the 900 records with the old company name, en masse. I don't want to have to scroll through each one manually.